Here's why:
* Abstract nouns refer to concepts, ideas, qualities, or states of being. Language is a system of communication, which is a complex concept. You can't physically touch or hold language.
* Concrete nouns refer to physical objects or things that you can see, touch, hear, smell, or taste. You can't physically interact with language in the same way you can with a book (a concrete noun).
Therefore, "language" is considered an abstract noun.